The present invention relates generally to a voice detector circuit, and more particularly to a circuit which detects the presence of a human voice in an audio signal and controls the operation of a system (such as a squelch circuit or a channel scanner) in accordance therewith.
Radio receivers commonly include circuits for determining when a modulated signal is present. Circuitry of this type is often included for the purpose of controlling a squelch gate for selectively passing the received signal to an output only when a modulated signal is present. Such circuits are also used in automatic channel-scanning radio receivers for controlling the operation of the channel scanner.
In conventional AM systems, detection of the modulated signal can be accomplished by the simple expedient of detecting the presence of the carrier signal. In single sideband systems, however, the carrier is not transmitted along with the modulated information; the presence of a modulated signal must therefore be determined by other means. A variety of systems have been devised for this purpose. Exemplary systems of this sort are described in the patents to Kemper (U.S. Pat. No. 3,350,650) and Eichenberger et al. (U.S. Pat. No. 3,102,236).
Although these circuits generally operate satisfactorily in low noise environments, it has been found that impulse noise commonly present in noise corrupted signals can interfere significantly with the operation thereof.
The present invention therefore provides a system for detecting a voice signal which includes improved noise immunity.
In accordance with the present invention, a voice detector circuit is provided which responds to an audio signal to provide an output indication of the presence or absence of a voice signal thereon. A slew-rate limited amplifier is provided at the input to a voice detector circuit for the purpose of removing impulse noise from the audio signal. The slew-rate limited amplifier is essentially transparent to ordinary audio signals, but greatly attenuates impulse noise components. The voice detector circuit operates on the impulse noise limited audio signal to provide the output indication.
In accordance with another aspect of the present invention, the voice detector circuit provides said output indication upon the basis of a comparison of the average syllabic-rate content of the audio signal with a threshold value.